Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Multiple transmit and/or receive chains are commonly used in many wireless communication systems for different purposes. Using multiple transmit and/or receive chains the spatial dimension can be exploited in the design of a wireless communication system. Wireless communication systems with multiple transmit and/or receive chains offer improved performance. The performance improvement can be in terms of better coverage, higher data rates, reduced Signal to Noise Ratio (SNR) requirements, multiplexing of multiple users on the same channel at the same time, or some combination of the above. Different techniques using multiple transmit and/or receive chains are often referred to with different terms such as Maximal Ratio Combining (MRC), Space-Time Coding (STC) or Space-Time Block Coding (STBC), Spatial Multiplexing (SM), Beam-Forming (BF) and Multiple Input Multiple Output (MIMO). Wireless communication systems with multiple transmit chains at the transmit entity and multiple receive chains at the receive entity are generically referred as MIMO systems.
In an SM-MIMO system a high data rate stream is split into multiple lower date rate streams and each lower data rate stream is transmitted from a different transmit antenna on the same frequency at the same time. Alternatively, data from two different users or applications may be transmitted from different transmit antennas on the same frequency at the same time. If signals from different transmit antennas arrive at the receiver antennas through sufficiently different spatial propagation paths, the receiver may be able to separate these streams of data, creating parallel spatial channels on the same frequency at the same time. SM is a powerful technique for increasing channel capacity at higher SNR. The maximum number of spatially multiplexed data streams is limited by the minimum of the number of antennas at the transmit entity and the number of antennas at the receive entity and the degree of spatial isolation between the antennas at the receive entity and transmit entity. For example if the number of transmit antennas at the transmit entity is four and the number of receive antennas at the receive entity is two, the maximum number of spatially separable data streams is two.
FIG. 2 illustrates an example of an SM-MIMO wireless communication system with four transmit chains at the transmit entity, for example a base station, and four receive chains at the receive entity, for example a client terminal. The signal from a transmit chain arrives at all four receive chains through different propagation paths as shown in FIG. 2. The receive signal at each receive chains may be a combination of signals transmitted from all four transmit chains and the noise as shown in FIG. 2.
SM-MIMO can be used in different configurations. Three different example configurations are shown in FIG. 3, FIG. 4, and FIG. 5. In the SM-MIMO configuration in FIG. 3 a single stream of data goes through the Cyclic Redundancy check (CRC) and Forward Error Correction (FEC) processing and then it is de-multiplexed into two streams for mapping to the two layers of the SM-MIMO transmission system. The SM-MIMO configuration in FIG. 4 is a multi-codeword MIMO system. In this configuration, the information from a single stream of data is first de-multiplexed into two streams and then the CRC and FEC are performed separately before mapping the two streams to the two layers of the SM-MIMO system. This method allows for independent decoding of the two streams of data and also allows for the Successive Interference Cancellation (SIC) type of receiver for improved performance. The SM-MIMO configuration in FIG. 5 is essentially the same as the configuration in FIG. 4 except that the two streams of data originate from two different users. This enables the ability to multiplex multiple users on the same resources at the same time and therefore it is often referred to as Multi-User SM (MU-SM). The SM-MIMO configurations in FIG. 3 and FIG. 4 are often referred to as Single User SM (SU-SM). In MU-SM mode, a single stream of data is transmitted for each user whereas in the SU-SM mode there are two streams of data transmitted to a single user.
In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system, different MIMO modes for transmission are defined. One such mode is the MU-MIMO where two users may be multiplexed on a single resource at the same time. The SM decoder for the MU-SM can be very similar to SM decoder for the SU-SM configurations shown in FIG. 3 and FIG. 4 if the information about the Modulation and Coding Scheme (MCS) is known for both the users. However, in 3GPP LTE wireless communication system, in order to reduce the overhead of control information in case of MU-SM, each individual client terminal multiplexed on to the same resource is aware of the MCS for the SM layer to which data for it are mapped. The client terminal may not be aware of the MCS used for transmission to other client terminals on the same resources. This method is also preferred for allocation flexibility where the multiplexing of multiple users on the same channel resources can be dynamic. For example, as shown in FIG. 6, portion of the channel resources, known as Resource Blocks (RB) in 3GPP LTE wireless communication system, are shared between client terminals A and B whereas another portion is shared between client terminals A and C and yet another portion may be used by client terminal A by itself, i.e., no MU-SM in that portion.
The number of possible MCSs used in 3GPP LTE wireless communication system may be high. For the client terminal to decode the complete payload information, it may need to know both the modulation type and coding scheme for the FEC decoding. On the other hand, for modulation symbol level decoding in an SM decoder, knowledge of only the modulation type is required. In most wireless communications systems, the number of modulation types is relatively smaller than the number of MCSs. However, after SM-MIMO decoding stage the data for the intended client terminal may be extracted and there is no need for the MCS of the other layer or codeword. Therefore, only requirement is that the SM-MIMO decoder be able to decode the signal intended for it without knowing the modulation type of the other layer or codeword.
In 3GPP LTE wireless communication system, the signal conditions may vary depending on the number of users, Signal to Interference and Noise Ratio (SINR) at different client terminals, the number and location of all the active client terminals, etc. It is desirable for the wireless communication network to be able to switch between non-SM, SU-SM and MU-SM modes without going through excessive signaling overhead. The 3GPP LTE wireless communication system provides a provision for such a dynamic allocation. Based on the provisions in the 3GPP LTE wireless communication system, in some scenarios the client terminal may be semi-statically configured to be MU-SM mode but may not be aware of whether there is actually another client terminal scheduled on the same resources at the same time. In conventional methods the decoding performance may be reduced for the case when the client terminal is in MU-SM mode but is unaware about the absence or presence of the scheduling of the other client terminal on the same resources at the same time. A method and apparatus are disclosed that enable improved decoding performance when the client terminal is unaware of absence or presence of the scheduling of the other client terminals when it is in MU-SM mode.